In order to increase or maintain the production from oil fields with a low production well head pressure (PWHP), a boosting system may be required. Such a system allows the pressure of the extracted gases and liquids to be boosted to the pressure required by downstream processing systems or for transportation by pipeline. If the pressure of these fluids cannot be boosted efficiently, it may be uneconomic to maintain production, or in the case of the multiphase fluids, it may be necessary to flare (burn off) the low pressure gas after separating it from the liquid phase.
In some situations it is known to separate multiphase fluids and use a mechanical booster pump to boost the pressure of the liquid phase, while a gas compressor is used to boost the pressure of the gas phase. However, gas compressors are very large and expensive pieces of equipment and this solution may not be viable where space is at a premium (for example on offshore oil production platforms) or where the available resources do not justify the necessary investment.
It is also known to use jet pumps (eductors) to boost the production of oil and gas wells. Jet pumps are simple, reliable, low cost devices that use fluids from a high pressure (HP) source to boost the pressure of fluids from a low pressure (LP) source. The high pressure fluids may for example be obtained from nearby high pressure wells, or from other available sources: for example it may be possible to use high pressure gas from a gas compressor or high pressure water that is used for injection into oil or gas wells to maintain pressure. Such systems work well, provided that a suitable high pressure fluid source is available.
An example of a system for boosting pressure that uses jet pumps is described in EP0717818A. This system uses fluids from high pressure wells to boost the production of adjacent low pressure wells.
EP1606492A describes another system, in which the gas and liquid phases of a low pressure fluid are separated, and high pressure gas from any available source (e.g. from a high pressure well, a lift gas supply or a compressor) is used to boost the low pressure gas phase using a jet pump, while a mechanical pump is used to boost the pressure of the liquid phase.
There are a number of limitations associated with the use of available HP motive fluids for jet pump systems. For example, a suitable high pressure fluid supply that can be used as the motive flow in a jet pump may be unavailable, or may be of inadequate pressure or flow rate. The high pressure fluid may not be sustainable in the long term, for example if it is drawn from another well. If high pressure water is used, the downstream processing system may not have sufficient capacity to cope with the additional fluid quantity. Furthermore, the addition of a motive fluid to the fluids produced from the well will increase the total quantity of fluid flowing through the transportation pipeline, leading to an increased pressure loss along the pipeline and thereby diminishing or negating the benefits gained by using a pressure boosting system.
GB2450565 describes a pressure boosting system that addresses some of these limitations. In this system a jet pump is used to boost the pressure of the LP fluid. The LP fluid may be either a multiphase fluid or a predominantly gas phase fluid, and the jet pump is driven by a HP liquid phase fluid that is supplied by a booster pump. The jet pump combines these fluids and the output of the jet pump therefore comprises a medium pressure multiphase fluid. This output fluid is delivered to a separator, which separates the gas phase from the liquid phase. Part of the liquid phase is then returned to the booster pump and used to drive the jet pump, while the remaining part of the liquid phase is recombined with the gas phase leaving the separator and then delivered to downstream processing systems. The system is self-sustaining, requiring no external supply of HP fluid to drive the jet pump.
One limitation of the system described in GB2450565 is that it cannot always cope well with large variations in the composition of the LP fluid drawn from the LP well. Such variations may occur for example when re-starting a well from which flow has ceased. When flow from such a well is re-started the flow may initially consist almost entirely of gas, later changing to a multiphase fluid with an increasing proportion of liquid as the flow increases and the well is brought back into production. In this situation it may be difficult to select a jet pump that operates efficiently under the changing flow regime.
Another problem can be encountered if the fluid drawn from the LP well contains a significant amount of sand. The sand tends to stay entrained within the liquid phase that is drawn from the separator and then returned to the booster pump and used to drive the jet pump. Any sand carried by the liquid phase will therefore pass through the booster pump and the jet pump. This can cause severe wear within those parts of the system, eventually requiring their replacement.